In the molding of thermoplastic material, the material after blending the various ingredients is generally formed into pellets. The blended material is fed in dry particulate form through an extruder which melts the material and forces the viscous molten material through a pelletizer. One type of extruder used for pelletizing thermoplastic material is made by BUSS-Condux, Inc. of Switzerland and includes a kneader having a screw which rotates and slightly reciprocates in a housing. The screw works the material and melts it as the material flows from the inlet end to the outlet end of the kneader. At the inlet end of the kneader is a hopper into which the material in dry particulate form is fed. The hopper contains a feeder screw to feed the material into the inlet end of the kneader. At the outlet end of the kneader is an outlet feeder screw which feeds the molten material from the outlet end of the kneader to a feeder screw which delivers the material to a pelletizer.
In the operation of this type of extruder, a zone of the molten thermoplastic material is formed at the outlet end of the kneader. To achieve uniform output from the extruder, it is desirable to maintain the depth of the molten zone uniform. If the depth of the molten zone increases, the larger amount of the viscous molten material in the zone increases the power required to turn the kneader screw. If the molten zone becomes too shallow, the power on the kneader screw may decrease and can cause some of the thermoplastic material to be fed to the outlet feed screw while still in dry particulate form and disrupt the proper pelletizing of the material.
If the thermoplastic material being pelletized is of uniform composition so that it has a uniform melting temperature, the extruder can be turned on so that each screw rotates at a predetermined speed and the material will pass through the extruder at a uniform rate. However, most thermoplastic materials contain fillers of various kinds. For example, a thermoplastic material used in making a capacitance electronic disc, such as is described in U.S. Pat. No. 4,228,050 to C. J. Martin et al., issued Oct. 14, 1980, entitled, "CONDUCTIVE MOLDING COMPOSITION" includes a mixture of polyvinylchloride resin, conductive carbon black particles, stabilizers for the resin and lubricants. The melting temperature of such a mixture may vary from batch to batch because of variations in the amount of each of the ingredients of the mixture; and can also vary even within a single batch if the ingredients are not thoroughly mixed. Even thermoplastic resins which do not include any fillers can vary in melting temperature from batch to batch because of weight average or number average molecular weight variations.
If there are variations in the melting temperature of the material being fed through the kneader, the viscosity of the material at the outlet end of the kneader will vary. This can cause undesirable variations in the depth of the molten zone which will vary the power requirements of the kneader screw. Heretofore, it has been the practice for an operator to watch for variations in the power requirements of the kneader screw, which occurs when the depth of the molten zone varies, and compensate for such changes by manually varying the speeds of the hopper feed screw, outlet feed screw and/or kneader screw. However, such manual control of the extruder is an art rather than a science and depends greatly on the skill and ability of the particular operator. Therefore, it would highly desirable to have means for automatically controlling the operation of an extruder to achieve a uniform output of the pelletized material.